Induction furnaces for melting or otherwise heating metal by generating magnetic fields which induce eddy currents to flow within and heat the metal are well known. One such induction furnace is the well-known "coreless" type having an induction coil assembly located external to the furnace itself. The induction coil assembly creates the magnetic flux which comprises the magnetic fields which, in turn, create the eddy currents to heat the metal. Typically, the metal to be heated by the furnace is contained by a liner which is of a refractory material. The eddy currents, induced by the magnetic fields generated by the induction coil assembly surrounding the liner, cause power (I.sup.2 R) to be dissipated in the metal, thereby increasing the temperature of the metal. In effect, for induction heating, the metal advantageously serves as its own heat source, thereby increasing the efficiency of the heating itself. The eddy currents are induced in the metal when alternating current is passed through the induction coil so as to generate an alternating magnetic field, or induction field.
The vessel in which the metal is heated must meet certain stringent physical standards. It must have a sufficiently high melting point so that it will not be melted by the heat of the metal, it must have a high strength to hold the weight of the metal, it must not interfere with the passage of magnetic flux from the induction coil through and around the metal, and in certain cases it must be removable from the induction coil assembly so that the melted metal within the vessel may be conveniently transported among various stations for pouring, holding, treating and other purposes.
The present invention is particularly well suited for heating vessels that are removable from induction coil assemblies. Removable heating vessels comprising a crucible are known and may be formed of a material, such as a ceramic. Ceramic, as is also known, is brittle and subject to stress cracking which may cause breaking of the ceramic, leading to "run out" of molten metal from the crucible. This "run out" poses a severe safety hazard to operating personnel. Thus, ceramic crucibles find little usage when the metal melted at one station needs to be transported in the same vessel to another station.
One way of strengthening a ceramic crucible is to surround it by a continuous metallic jacket, or shell typically of a metal having a relatively high temperature characteristic. However, since this type metal is either electrically conductive, magnetic, or weakened when heated, metallically supported ceramic crucibles by themselves do not offer much of an improvement over ceramic crucibles since the magnetic field generated by the induction coil assembly will heat the shell thereby reducing its mechanical integrity, while at the same time diverting energy away from the heating of the material. The magnetic field commonly causes a power loss (I.sup.2 R) while creating this self heating of the shell.
Thus there exists a definite need for a mechanically metal-jacketed induction heating vessel that overcomes the drawbacks of undesirable self heating of the jacket and the accompanying diversion of energy from the heating of the metal. The present invention provides such a heating vessel that does not divert energy away from heating the metal while at the same time does not suffer from any unnecessary self heating that might otherwise degrade its mechanical integrity. Further, the structurally rigid heating vessel of the present invention is easily removed from the induction coil assembly so that it may be conveniently transported between stations. The metallic shell that provides the structural support of the vessel is arranged in a predetermined manner relative to the induction coil assembly so as to obtain the benefits of the present invention. The metal-jacketed induction heating vessel of the present invention, commonly termed a ladle, is able to handle large quantities of metal at high operating temperatures. Furthermore, the ladle of the present invention, having its attendant benefits, is particularly suited for vacuum induction furnaces.
Typically, induction furnaces are provided with means for cooling the coils of the induction coil assembly and/or means for cooling the liner containing the metal. Sometimes, either or both types of cooling means are located where they intercept the electromagnetic fields generated by the induction coil assembly and, thereby as previously discussed, disadvantageously absorb or divert the flux of the magnetic fields away from its intended purpose of heating the molten metal. It is desirable to avoid cooling means located such that they interfere with the electromagnetic fields, so as to improve the heating efficiency of the induction furnaces.
All induction furnaces comprise a liner formed of a refractory material that contains molten metal within the furnaces. This refractory liner may have to be replaced sometime during the life of the induction furnace. It is desired that induction furnaces be provided with means that allows for easy access to and replacement of the refractory liner.
Accordingly, it is an object of the present invention to provide induction furnaces having means that facilitate the replacement of its crucible.
It is a further object of the present invention to provide induction furnaces having an induction coil assembly for generating, distributing and directing magnetic fields used for the heating of the metal in an improved manner, and a ladle that is readily removable from the induction coil assembly and has a metallic shell that allows the generated magnetic fields to easily pass therethrough without any substantial interference, so that the magnetic field will not unnecessarily cause the heating of the shell, but rather be more advantageously delivered to the metal contained within the crucible.
It is a further object of the present invention to provide an improved coil assembly and a ladle, having a metallic shell, for all types of induction furnaces including a vacuum type.
Further still, it is an object of the present invention to provide induction furnaces having means for removing the heat dissipated by the induction coil assembly without interfering with the electromagnetic fields generated thereby.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.